yes, i installed the latest 13.1 AMD drivers today and just restarted my computer, the problem still persists.
the plugins are the official Nvidia texture tools plug ins, which is a filter, and an exporter
i removed the plugins, and the problem still persists.

how the problem appears:
i zoom in, i go to one of the edges, so about half the screen shows the outside of the canvas. i draw a line, it draws at normal speed. i press ctr+z, and draw again. it draws at normal speed.
HOWEVER.
if i click in the history tab, drawing speed immediately slows down significantly to a crawl.
to make it speed up, i need to zoom out until the entire canvas is in display.
there's a slight lag, and then it will draw at normal speed again.
it slows down all drawing brushes.
i'm working on a 2048x2048 texture with 39 layers, and about 15 layers groups or so.

Same problem with Win8 x64, 6gb ram, Ati hd6950, Catalyst 13.x (beta too).
With a previous installation of Win8 x64 with Catalyst 12.10 i had no lag.
Disabling the graphic acceleration, all works good.